I've just come across a case in Oracle 8.0.6 where important queries
could have been several orders of magnitude faster if only the optimizer
had realized that it was doing case-insensitive comparisons against a
constant that wasn't affected by case (a string of all digits).
The query was of the general form
SELECT * FROM table
WHERE upper(id) = '001234'
...causing a full index scan (there was a non-unique index on id). What
the optimizer could perhaps have done was something like
if (upper('001234') == lower('001234'))
SELECT * FROM table
WHERE id = '001234';
else
SELECT * FROM table
WHERE upper(id) = '001234';
Even without the index I guess that would have saved it a lot of work.
In this case, of course, the user wasn't doing the smartest thing by
giving millions of records a numerical id but storing it as varchar.
OTOH there may also be a lot of cases like
SELECT * FROM table
WHERE upper(name) LIKE '%'
being generated by not-too-bright applications out there.
Does PostgreSQL do this kind of optimization? If not, how easy and how
useful would it be to build it? I suppose this sort of thing ought to
be in src/backend/optimizer/prep/ somewhere, but I couldn't find
anything like it.
Jeroen